Program version (important because some versions have different log formats; This describes 3.3.x and later log format. One important thing to note is that the 3.2.2 and earlier had different date formats and were not Y2K compliant.)

(The information can be found from the Nethack source code, of course, but when finding out the information for this writeup and experimenting with the parsing myself (this turned into one monstrously big but well-working Java app), Jukka Lahtinen's small C program for analyzing logfiles helped somewhat...)